It is desirable for a tire to exhibit good traction characteristics on wet and dry pavements, and for the tire to provide good treadwear and low rolling resistance. In order to reduce the rolling resistance of a tire, rubbers having a high rebound can be utilized in making the tires' tread. Tires made with such rubbers undergo less energy loss during rolling. The traditional problem associated with this approach is that the tire's wet traction and wet skid resistance characteristics are compromised. This is because good rolling resistance which favors low energy loss and good traction characteristics which favor high energy loss are viscoelastically inconsistent properties.
In order to balance these two viscoelastically inconsistent properties, mixtures of various types of synthetic and natural rubber are normally utilized in tire treads. For instance, various mixtures of styrene-isoprene rubber (SBR), polyisoprene rubber, and natural rubber are commonly used in automobile tire treads formulations. Styrene-isoprene rubber is included in tire tread formulations primarily to improve the traction characteristics of the tire without greatly compromising tread-wear or rolling resistance.
The versatility of solution SBR (SSBR) synthesis relative to the synthesis of emulsion (ESBR), including control of molecular weight, macrostructure, microstructure, and functionalization, is well established (see Hirao, A.; Hayashi, M. Acta. Polym. 1999, 50, 219-231, and references cited therein). Performance advantages arising from this versatility have led to an acceleration of the replacement of emulsion SBR in the fire industry, and an expansion in the market for random, low vinyl SBR for use in tire compounds (see Autcher, J. F.; Schellenberg, T.; Naoko, T. "Styrene-Butadiene Elastomers (SBR)," Chemical Economics Handbook SRI-International, November, 1997). These developments have stimulated interest in developing technology for commercial production of random, low vinyl solution SBR.
Although anionic initiated synthesis of random medium vinyl solution SBR and random high vinyl solution SBR is easily accomplished by the addition of Lewis bases, these polar modifiers promote randomization at the expense of increased vinyl content (see Antkowiak, T. A.; Oberster, A. E.; Halasa, A. F.; Tate, D. P. J. Polym. Sci., Part A-1, 1972, 10, 1319). Due to the large differences in monomer reactivity ratios of isoprene and styrene, measures must be taken to promote random incorporation of styrene into low vinyl solution SBR In the absence of such measures, the polymerization leads to a tapered block copolymer with inferior elastomeric performance characteristics (see U.S. Pat. No. 3,558,575).
British Patent 994,726 reports that it is possible to produce random solution SBR by manipulating monomer polymerization rates via control of monomer concentrations throughout the polymerization process without the use of polar modifiers. For solution SBR, this requires that the polymerization proceed in a styrene rich medium throughout the polymerization. In continuous polymerizations the issues associated with maintaining constant monomer concentration ratios while increasing conversion become quite complex.
U.S. Pat. No. 3,787,377 reports that alkali metal alkoxides (NaOR) can be used as polar modifiers in the copolymerization of styrene and isoprene to randomize styrene incorporation without significantly increasing the vinyl content of the rubber. However, alkali metal alkoxide modifiers are so effective that they may actually increase the rate of polymerization of styrene to the extent that it is depleted before the polymerization is complete (see Hsieh, H. L.; Wofford, C. F. J. Polym. Sci., Part A-1, 1969, 7, 461-469). Furthermore, there is typically some undesired increase in vinyl content over what would be expected from an unmodified polymerization (see Hsieh, H. L.; Wofford, C. F. J. Polym. Sci., Part A-1, 1969, 7, 449460).